ergophobe - 7:11 pm on Feb 28, 2012 (gmt 0)
@Pfui - robots.txt is not applicable here. Pinterest isn't crawling anywhere.
@g1smd and @Marshall - g1 - I think Marshall has your answer. This provides an answer to pro photographers and Getty and so forth.
Frankly, if you are not in the business of charging people to use your images, I think your best bet is to watermark images with your domain name and hope that people share them everywhere they can.